The present invention relates generally to fiber optic couplers, and more specifically to single mode to multimode fiber optic directional couplers.
Fiber optic directional couplers are used in communication networks, optical signal processing, and sensor systems. For example, fiber optic local area networks (LANs) utilize fiber optic couplers to coupler light between a fiber bus and plural work stations, each of which comprises an optical transmitter and receiver.
Most prior art directional couplers are "reciprocal" devices, that is, they inherently couple the same fraction of optical power regardless of whether the coupling is from a first fiber to a second fiber or from the second fiber to the first fiber. Non-reciprocal coupling devices, on the other hand, couple significantly different fractions of optical power between fibers. Such non-reciprocal coupling devices are more advantageous than reciprocal coupling devices for many applications, such as local area networks. For example, in a local area network, it is desirable to couple only a small fraction of the data signal optical power from the bus to each station, so as to permit the bus to accomodate a large number of stations. On the other hand, it is desirable that substantially all of the data signal optical power generated at each station be coupled to the data bus. Since reciprocal couplers couple the same amount of optical power in both directions (i.e., bus to station or station to bus), the use of such reciprocal couplers in local area networks severely limits the maximum number of stations. Similar problems exist in other fiber optic applications, such as multiplexed sensor networks.
The foregoing problems have been addressed in the prior art by utilizing a nonreciprocal, single mode to multimode fiber optic directional coupler. The multimode fiber serves as a data bus, while the single mode fiber serves as a network branch, e.g. for a work station. Optical data generated at the work station is transmitted through the single mode fiber for coupling to the multimode fiber bus. The prior art couplers are typically formed by removing a portion of the cladding from the side of a single mode fiber and a multimode fiber, and then juxtaposing the portions of the fibers where the cladding has been removed, so that the fibers are in side by side relationship. However, to ensure effective coupling between the single mode and multimode fiber, the coupled modes must be phase matched so that they have approximately the same phase velocity. Since the modes of a multimode fiber have phase velocities spread over a substantial range, only a limited number of higher order modes within the multimode fiber are utilized. The resulting mode distribution in the multimode fiber is, therefore, not uniform, and is likely to be influenced by microbends or other fiber perturbations. Accordingly, the coupling coefficient of such a coupler may be influenced by the environment.